1. Field of the Invention
This invention relates to molded articles having excellent optical characteristics. More particularly, it relates to molded articles having excellent transparency and a reduced degree of optical strain and also to a method for producing such molded articles.
2. Description of the Prior Art
Molded articles used for optical purposes in the form of, for example, plates, sheets, boxes, blocks or other various shapes should not only be transparent, but also have a reduced degree of optical strain. Especially, digital signal recording discs such as digital audio discs, digital video discs and signal recording discs for electronic computers should have excellent transparency and small optical strain. For instance, for digital audio discs having a thickness of 1.2 mm and a diameter of 120 mm, the retardation is needed to be not more than 60 m.mu., preferably not more than 40 m.mu.. In addition, the warpage should be so small that for the digital audio disc, it is needed to be not more than 0.2 mm, preferably not more than 0.1 mm.
The manufacture of these molded articles is conveniently effected by molding polycarbonate resins with excellent transparency by the injection molding technique. However, in order to obtain moldings having a reduced degree of optical strain, it is necessary to permit the resin to flow smoothly at the time of the injection molding so that the flow of the resin melt does not cause optical uneveness.
The good flow of the resin melt may be attained by a method in which the injection molding is effected at high resin temperatures. However, high resin temperatures are disadvantageous in that the resin may be colored or yellowed such as by decomposition of the resin, impeding the transparency of the resultant molding. Alternatively, polycarbonate resins having low molecular weights may be used. However, such polycarbonate resins, e.g. polycarbonate resins having an average molecular weight less than 12000, considerably lower in molecular weight when injection molded, leading to the disadvantage in that their mechanical properties greatly lower and the resultant molding will break upon release of the molding.
We have made the intensive study on a method for obtaining resin moldings having excellent transparency and a small degree of optical strain and found that polycarbonate resins which contain a phosphorous ester and have a certain range of average molecular weights exhibit excellent melt flowability and excellent heat stability. These resins can provide transparent molding which are less strained optically when injection molded under certain conditions.